1. Field of the Invention
This invention relates to a system for preventing the wheels of a motor vehicle from skidding when braked, and more particularly it pertains to an anti-skid control system for motor vehicles, which is so designed as to be able to most effectively cope with a situation where the coefficient of friction of the road surface suddenly changes from high to low during anti-skid control operation.
2. Description of the Prior Art
Generally, with an anti-skid system for motor vehicles, hold valves and decay valves comprising electromagnetic valves are opened and closed on the basis of electrical signals representing wheel velocities sensed by means of wheel velocity sensors, thereby increasing, holding or reducing the brake pressure, for the purpose of securing improved steering performance and running stability of the motor vehicle during braking operation, while at the same time shortening the braking distance.
FIG. 1 of the accompanying drawings illustrates, by way of example, manners in which wheel velocity Vw and brake pressure Pw are varied during the operation of a conventional anti-skid control system, together with a hold signal HS and a decay signal DS for opening and closing hold valves and decay valves, respectively.
When the brake apparatus of a running motor vehicle is not operated, the hold valves remain open while the decay valves remain closed, and the brake pressure Pw is not increased. When the brake apparatus is operated, the brake pressure Pw is increased so that the wheel velocity Vw is decreased. As the wheel velocity Vw is being decreased, and when the deceleration (negative acceleration) of the wheel reaches a predetermined level, the hold signal HS is generated, whereby the hold valves are closed so that the brake pressure Pw is held constant; and at a point of time when the wheel velocity becomes equal to a reference wheel velocity Vt which is set up to follow the wheel velocity Vw with a predetermined relationship thereto, i.e., at time t1 when curves representing the wheel velocity Vw and reference when velocity Vt cross each other (see FIG. 1), the decay signal DS is generated, whereby the decay valves are opened so that reduction of the brake pressure is initiated. As a result of this reduction of the brake pressure Pw, the wheel velocity Vw is changed from decreasing to increasing, at time t2 when a low peak Vl of the wheel velocity Vw occurs. Either at the time t2, or at time t3 when the wheel velocity has been increased up to the level of a velocity Vb that is higher than the low peak by 15% of the difference A between the wheel velocity Va at the time t1 when the reduction of the brake pressure was started, and the low peak velocity Vl, the decay signal is interrupted so that the reduction of the brake pressure Pw is interrupted and thus the brake pressure Pw is held constant. At time t4 when the wheel velocity Vw reaches the level of a high peak velocity Vh, buildup of the brake pressure is again started. In this case, the buildup of the brake pressure is effected in such a manner that the brake pressure is alternately increased and held constant in succession due to the fact that the hold signal HS occurs mincingly, so that the brake pressure Pw gradually builds up; in this way, the wheel velocity Vw is decreased, and the mode for reduction of the brake pressure occurs again.
During the operation of the aforementioned conventional anti-skid control system, if the coefficient of friction of the road surface changes from high to low, i.e., if a so-called "high jump" road surface condition occurs, then the wheel velocity Vw will be reduced sharply; thus, with the above-described conventional arrangement wherein reduction of the brake pressure is simply stopped either at time t2 when a low peak of the wheel velocity Vw occurs or at time t3 when a 15% increase in the wheel velocity Vw from the low peak occurs, the wheel velocity Vw tends to be behind in its recovery so that one or more of the wheels tend to be subjected to early-locking.